Light waveguide data communications (also referred to here as optical data communications) is becoming increasingly popular due to its advantages in relation to systems that use conductive wires for transmission. Such advantages include resistance against radio frequency interference and higher data rates. An example of a light waveguide transmission system is an optical fiber cable link. Such links are widely used for high speed communications between computer systems. Each system that is attached to the link has a transmitter portion and a receiver portion. The transmitter portion includes electronic circuitry that controls a light source such as a laser, to generate a light signal in the cable that is modulated with information and/or data to be transmitted. The light signal is detected at the receiver portion by a light detector, such as a photodiode, and with the help of appropriate circuitry the received data is then demodulated and recovered.
The transmitter and receiver portions of an optical link are designed with the concept of an optical power budget in mind. The required power or light intensity of the source signal at the transmitter is a function of not just the dynamic range of the receiver, but also connector losses and fiber attenuation. Enough power should reach the receiver such that the signal to noise ratio (SNR) is adequate to achieve a minimum bit error ratio (BER). In other words, the transmit power needs to be high enough so that despite such losses, there is enough signal power at the receiver to detect the transmitted information. The higher the losses in the link, the smaller the available light intensity range in which the data to be transmitted can be modulated or encoded. In addition, optical fiber cable causes dispersion in the light signal, making it difficult for the receiver to distinguish between adjacent data symbols in a received sequence. The data symbols are conventionally encoded using a binary coding scheme where each symbol that is transmitted is represented by one of only two different light intensity levels.